Multilayer structures are typically formed from a core layer sandwiched between two face sheets. The core layer of a multilayer structure defines substantial air/void space and has an effective cross-sectional thickness that is substantially greater than the cross-sectional thicknesses of the adjacent face sheets. Therefore, multilayer structures typically possess relatively high strength and stiffness at relatively low weight. As such, multilayer structures are used in various aerospace applications.
Superplastic forming is a known technique for manufacturing multilayer structures, such as expanded two-sheet panels (no core layer) and expanded three-sheet panels (a core layer positioned between two face sheets), which may be used as alternatives to traditional honeycomb multilayer structures. Superplastic forming is a metal forming process that takes advantage of the superplasticity of certain materials, such as titanium alloys, aluminum alloys and nickel alloys, at elevated temperatures. When such materials are heated to a superplastic state, they become pliable and can be expanded (e.g., by gas pressure) against a mold to achieve a desired shape. During expansion, the material can experience elongation of several hundred percent.
While the air/void space in multilayer structures advantageously reduces density (increases bulk), it presents a complication when a multilayer structure must be connected to another structure (e.g., a frame or another multilayer structure). For example, inserting a mechanical fastener, such as a bolt, through a multilayer structure and then tightening down on the multilayer structure may cause collapse of the air/void space, which may compromise the associated connection.
Accordingly, those skilled in the art continue with research and development efforts in the field of multilayer structures.